Aircraft: B737
Phase of flight: Other Holding Pattern
Anomaly: ATC Issue All Types; Airspace Violation All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence

I returned to the area and was asked to open two sectors which were combined at a third sector. By this time the sector was already out of control. Numerous arrivals were holding at various fixes in the sector. Many of these aircraft had incorrect information in their flight plans and in the 4th line of the data block. Hold messages had not been entered for many of the aircraft which had been assigned holding. Because the controller who had been working the sectors was already overwhelmed I did not receive an adequate briefing. As a result I was basically left to figure out what was going on in a very chaotic situation with information that was either incomplete or incorrect. I didn't even know which aircraft had been assigned holding or what fix they had been assigned to hold at. Many of these aircraft had also been left at high altitudes. There were aircraft holding as high as 38;000 ft. over the approach gate. Having these aircraft so high; so close to the gate added more issues as several eventually requested divert which meant I had to vector aircraft around numerous holding patterns in order to descend. There was weather in the sector that was not only causing deviations for en route aircraft; but also causing some of the aircraft in holding patterns to make turns in opposite direction from published to stay clear of weather. The CS (Combine Sector) message was entered by the D-side incorrectly which added to workload as I had to find and steal tracks that were in my airspace but had auto-flashed to an incorrect sector. The workload at the sector also caused delay in moving aircraft to my frequency at the newly opened sectors which impeded my ability to correct situations that were already in progress when I arrived. Because hold messages had not been entered many of the tracks progressed which resulted in automation issues; so workload was further increased as we had to update flight plans and either force them to approach control or make manual handoffs as we exited holding.There was also poor coordination of the exit from holding itself. At numerous times I received conflicting information from the approach control; Supervisor; and TMU (Traffic Management Unit) about when I could move aircraft on; how many aircraft I could move on; what route or altitude approach control required; the runway in use for descend via procedures; etc. The constant changes as well as conflicting information I was receiving added greatly to the complexity at the sector. At one point during the session an aircraft deviated into my airspace without approval and caused a conflict with an aircraft in holding. This situation required my immediate attention to resolve and hindered my ability to bring the sector under control. As I was trying to sort all of this out I missed Aircraft X in conflict with Aircraft Y. When I observed this conflict my attempt to resolve it was inadequate; largely because I was unsure where Aircraft X was assigned holding and which direction they were making turns. Aircraft Y responded to an RA.The sector should not have been allowed to get to the point it was at when I arrived. Part of the reason this happened was no-notice shut off of arrivals before I returned to the area. We need some procedure for holding notification that allows time to transition smoothly; and approach control should be taking some of the aircraft for holding at the published patterns within their airspace rather than refusing any traffic. I was told after the session that the Supervisor had been trying to page me. While I disagree with his statement that the situation would have been better if I had arrived just a few minutes earlier; I did not hear the page; and have noticed that our paging system is difficult or impossible to hear at some areas on the grounds and should be upgraded. I also feel that the airspace/holding patterns in the NE arrival gate are poorly designed. The airspace is too narrow to hold aircraft on parallel arrivals at the same altitude or to make opposite direction turns at the published holding patterns if there is weather or traffic.This tight space combined with frequent no notice shut off of aircraft on discretionary 'descend via' descents limits controller options. I also feel that there is a cultural problem with airspace violation at this Center. As I mentioned there was an aircraft that deviated into my airspace without approval and caused a conflict which took my attention away from other situations I was dealing with. The controller had issued an automated point out but I had not observed it until the aircraft was already entering my airspace and in conflict. This happens frequently and I believe there is a lack of understanding that the transferring control is still required to verbally coordinate if automated hand offs or point outs are not addressed before the aircraft enters the receiving controller's airspace. Center Controller reported an airspace violation and traffic conflict while opening a sector from one Controller working three sectors which were unmanageable due to weather deviations and volume.